Allows post-processing code to exclude recursing into host_build subdirs.
The alternative would be to have the SUBDIRS logic pre-parse the subdir
project to check if it's a host_build, but that might have a performance
impact, so it's better to leave the information explicit in the subdir
project file.

Change-Id: I468ceeaedce54b13bf672f82b9dcf04cc19d15e1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>